Supracrustal rocks, polymetamorphism, and evolution of the SW Greenland Archaean gneiss complex.

The complex contains the approx 3800 m.y. Isua supracrustal sequence of metavolcanic amphibolite, and chemical and clastic metasediments. Stratigraphic relationships show similarity to upper portions of Archaean greenstone belts, whereas geochemistry of the clastic sediments may record progressive unroofing of a volcanic source terrain. The rocks underwent amphibolite-grade metamorphism (T approx 550oC, P approx 5 kbar) before approx 3600 m.y. following emplacement of approx 3700 m.y. Amitsoq orthogneiss. The approx 3000 m.y. Malene supracrustal suite consists of metavolcanic amphibolite, minor carbonate and calc-silicate, cordierite-orthoamphibole gneiss and clastic metasediment, possibly deposited in a continental margin environment on a basement of mafic oceanic crust and older Amitsoq gneiss. Amphibolite- to granulite-grade metamorphism at approx 2800 m.y. preserves a kyanite rt arrow sillimanite transition, with T approx 550-850oC, P approx 5-9 kbar, accompanied by formation of Nuk orthogneiss. Retrogression to greenschist/amphibolite grade at approx 2650 m.y. appears related to shear zone formation. The kyanite present suggests no substantial P decrease. Mineral assemblages and inferred P-T conditions indicate a Barrovian-type environment for Isua and Malene metamorphism; crustal thicknesses >15 km before 3600 m.y. and >25 km before 2800 m.y. are indicated; P-T relationships indicate a thermal gradient of approx 30-40oC/km; widespread retrograde metamorphism may follow a transition from regional flow folding to deformation in discrete zones that provided pathways for fluid migration during the final stages of continent stabilization (Nuk gneiss). (Author's abstract)-C.N.